ABSTRACT The hematopoietic subsyndrome of acute radiation syndrome (H-ARS) is induced with even moderate exposure to ionizing radiation, which is lethal to hematopoietic stem cells (HSC) in the bone marrow (BM). Hematopoietic stem cell transplant (HSCT) is a potential life-saving treatment for H-ARS victims; however, it is not generally recommended due to commonly associated complication of graft-versus-host disease. Recent clinical trials, including that of our collaborator, have demonstrated that mesenchymal stromal/stem cell (MSC) infusions prevent/treat graft-versus-host-disease (GVHD) and reduce graft failure following HSCT. These salutary effects of MSC are due to potent immunomodulatory properties of the cells. Furthermore, MSC possess many other therapeutic properties shown to ameliorate pathologies associated with other ARS syndromes effecting the gastrointestinal system, skin and liver. Thus, MSC have the potential to treat the full spectrum of pathologies associated with ionizing radiation exposure. Ossium Health is developing a unique source of BM-derived multipotent MSC obtained from deceased organ donors. This cellular therapy, termed OssioStem-M3 (matched multipotent MSC) has a distinct advantage over other MSC sources in that it is donor-matched to a source for HSCT grafts (OssioStem-HSC) obtained from the same donors. Additionally, the large volumes of BM that can be obtained from a deceased donor yield exceptionally high numbers of MSC. This is important given that efficacy of MSC for supporting HSCT is negatively related to passage number; thus, low passage OssioStem-M3 is superior to ?third party? MSC obtained from living donors which must be amplified through many passages. We propose here to optimize methods for isolation of OssioStem-M3 and thoroughly characterize their physical characteristics and biological function. We also propose to demonstrate efficacy in large (porcine) and small (mouse) models of H-ARS. Finally, we will build and test next generation devices for increasing the consistency and throughput of OssioStem-HSC production. The overall product of this research program will be a robust production process and a compelling preclinical package to justify definitive studies to support FDA approval for H-ARS under the Animal Rule.